The term “crystalline hybrid nanoporous material” used in the present invention refers to, unlike the conventional zeolite, a molecular building block which is formed by coordinating an organic ligand or an organic ligand and an anionic ligand to or with a metal ion or a metal ion cluster to which oxygen is bonded. The term of crystalline hybrid nanoporous material is also used to refer to a structural skeleton that can infinitely expand with forming a void space when the 3-dimensional framework is formed
In particular, the term of crystalline hybrid nanoporous material can be defined as a porous organic-inorganic high molecular compound formed by bonding central metal ion(s) with organic ligand(s), and can mean a crystalline compound containing both organic moiety and inorganic moiety in the structural skeleton and having molecule-sized or nano-sized pores.
Recently, crystalline hybrid nanoporous materials are newly developed by the fusion of the molecular coordinate bond and the material science. Since said crystalline hybrid nanoporous materials have a high surface area and molecule-sized or nano-sized pores, they can be employed as an adsorbent, a gas-storing material, a sensor, a membrane, a functional thin film, a drug-delivering material, a catalyst, a catalyst carrier, etc. In addition, an intensive research on crystalline hybrid nanoporous materials is actively performed in recent since these materials can be used for collecting guest molecules which are smaller than the nano-pores or selectively dividing molecules depending on the size thereof by using nano-pores.
Further, the crystalline hybrid nanoporous material simultaneously contains polar parts such as metal ion and anionic ligand as well as non-polar parts such as aromatic compound group in its crystalline skeleton, by which it can possess both the hydrophilic property and the hydrophobic property.
If a crystalline hybrid nanoporous material has a permanent porosity, it can have a potential applicability in various fields, and thus, this matter has been a main concern in the fields of a catalysis, an absorbent and/or adsorbent, an ion exchange material, a material for chromatography, a storage material and a material for freshwater production, etc. Such solid porous materials can now be found in U.S. Pat. Nos. 7,855,299 and 6,893,564, etc.
However, such novel crystalline hybrid nanoporous materials are based on a skeleton or framework of metal-organic compound and are generally obtained in the form of small crystal or powder, which makes the application inconvenient because the density is low and the size is small in such form. Further, such powder form makes the recovery difficult and the deactivation and agglomeration easy. Therefore, the permeability of gas and liquid is not good, and thus, it cannot be easily utilized in most fields. In order to enable the crystalline hybrid nanoporous material to be utilized in various fields, various forms such as a composite are now considered and searched.
U.S. Pat. No. 7,524,444 and a literature [Micropor. Mesopor. Material. 2009, 120, 221] describe the method of converting a crystalline hybrid nanoporous material into a composite such as pellet. In said literatures, the composite is prepared by a molding step including compacting or extruding of the powder of metal-organic frameworks. However, such method has a problem that it may cause the decrease in the surface area and the porosity of the nanoporous material.
Meanwhile, conventional inorganic-type nanoporous materials can be shaped by employing an organic binder or an additive as a void-forming agent during the shaping step and then removing it by the calcination at a high temperature to generate voids between particles. However, since crystalline hybrid nanoporous materials are low in the thermal stability and thus cannot be subjected to the calcination, it is not easy to provide voids by simply adding an organic additive.
Further, since a crystalline hybrid nanoporous material has features such as the high crystallinity and high porosity on the basis of a skeleton consisting of metals and organic materials, the mechanical stability is low, the skeleton structure can be easily destroyed and the porosity features such as surface area can be easily reduced. As to the crystalline hybrid nanoporous material in which unsaturated metal sites can be easily formed in the structure during the removal of water or solvent, the preparation of a composite by compacting or extruding the powder will cause the reduction of active surface due to the loss of the unsaturated metal sites.
Therefore, the most important point in converting a crystalline hybrid nanoporous material powder to a composite having a certain shape is to minimize the deterioration of the features such as the surface area, the porosity and the active surface of the nanoporous material.
Another important point in converting a powder to a composite is the stability or the hardness of the composite. The stability of a composite is generally related to the pressure employed when shaping the composite. The hardness of a composite is closely related to the stability of the composite. A stable composite may be desirable on one hand, but on the other hand, the pressure employed when shaping the composite may reduce the surface area and the active surface.
Meanwhile, since inorganic nanoporous materials such as zeolite are based on the crystalline or amorphous skeleton consisting of metals and inorganic materials, they have a high stability but a low collectivity, and thus, it is necessary to mix them with a binder or an additive and subjected to a compacting under a mechanical pressure. The shaping of zeolite or the like by a compacting method can be found in US Patent Publication No. 2009-0048092, US Patent Publication No. 2011-0105301, etc.
Consequently, when a composite is prepared by a compacting manner, the surface area of the composite will be drastically reduced, when comparing with the surface area of the powder, by the decrease due to the amount of the binder or additive added, as well as by the decrease due to the blockage of the pore entrance, by the decrease due to the reduction of crystallinity, etc.
In the shaping of nanoporous materials, it should be noted that, since the active surface of the nanoporous materials is present at the inside the nanopores, it is important to ensure the void volume, in other words, to secure the channel or passage through which guest molecules can rapidly and smoothly penetrate from the outer surface to the inner nanopores of the shaped body. As to conventional inorganic nanoporous materials such as zeolite, they have been commonly shaped by the method for forming voids by using an organic binder, wherein the inorganic nanoporous material is mixed with an organic binder, shaped to form a composite, and then calcinated at a high temperature to remove the organic binder and to form voids.
Thus, there is still a need for a composite containing a nanoporous material powder and a method for the preparation thereof, in which the surface features of the powder can be maintained and voids can be formed when converting the powder to the composite, as well as the composite possesses a sufficient stability and the decrease of the active surface can be minimized to maximize the applicability of the composite.